The present invention relates to an optical displacement measurement apparatus, more particularly relates to a high precision optical displacement measurement apparatus using a photoelectric transmission type linear encoder, used for a contact type digital displacement meter.
In the past, optical measurement devices using lasers or light emitting diodes (LED) and optical measurement devices using optical encoders have been known. Optical measurement devices can achieve a high precision since they use as units of measurement the wavelengths of the lasers or LEDs. Further, optical measurement devices are mainly used for measuring the length between two points, i.e., measuring relative position. Optical encoder measurement devices are comprised of a scale made of a glass plate, film, metal sheet, etc., an optical grid provided at a predetermined pitch from the scale, a fixed index grid arranged facing the scale across a predetermined distance (the phase of the optical grid and the phase of the fixed index grid being shifted 90 degrees), a fixed light source for emitting parallel light to the scale, and a light receiving sensor. When the scale moves, the optical grid and fixed index grid overlap with each other to produce differences in lightness are produced. The light receiving sensor detects the difference in lightness. Optical encoder measurement devices are being used commercially as digital displacement meters and are mainly being used for measuring the length between two points, i.e., measuring relative position.
Below, optical encoder measurement devices of the prior art will be explained with reference to the drawings.
FIG. 1 shows the state of use of a contact type digital displacement meter 40 including a photoelectric transmission type linear encoder. The contact type digital displacement meter 40 is used connected to a counter 41 by a connection cable 7. The contact type digital displacement meter 40 is supplied with power from the counter 41 to perform measurement and outputs the measurement value to the counter 41. The counter 41 processes the signal output from the contact type digital displacement meter 40 and digitally displays the obtained measurement value on a display unit 42. Therefore, the displacement of an object measured by the contact type digital displacement meter 40 is displayed on the display unit 42 as a digital value.
The contact type digital displacement meter 40 has a frame 8 covered by an upper cover 9A and a lower cover 9B and has bearings 18 fastened at the two ends of the frame 8. The bearings 18 support a spindle 5. A contactor 6 is screwed into the front end of the spindle 5.
When measuring the length, thickness, etc., the contactor 6 screwed into the front end of the spindle 5 is brought into contact with the measured object. Displacement of the measured object causes the spindle 5 to move in the arrow direction. The displacement of the spindle 5 is detected by the photoelectric transmission type linear encoder built into the contact type digital displacement meter 40, the detection output is processed by the counter 41, and the displacement of the measured object is displayed on the display unit 42.
FIG. 2 explains the principle of a conventional photoelectric transmission type linear encoder built into the contact type digital displacement meter 40 explained in FIG. 1. The spindle 5 into which the contactor 6 is screwed has connected to it a moving scale 3 made of a transparent member. The moving scale 3 is formed with an equal pitch optical grid 11.
At one side of the moving scale 3 are provided a light source 1 and a condenser lens 2. At the other side are provided a fixed scale 34 formed with equal pitch optical grids 47 and 48 and light receiving elements, i.e., photodiodes 28. The light source 1 and the photodiodes 28 face each other across the moving scale 3 moving in accordance with displacement of the measured object and the fixed scale 34 fixed to a constant position.
The optical grid 11 provided at the moving scale 3 and the optical grids 47 and 48 provided at the fixed scale 34 have the same pitches and same line widths, for example, pitches of 20 xcexcm and line widths of 10 xcexcm. The two types of scales are fabricated to extremely high precisions.
At the time of measurement, the spindle 5 moves in the arrow direction. The amount of light passing through the scales becomes maximum when the transparent portions of the optical grid 11 of the moving scale 3 and the transparent portions of the optical grids 47 and 48 of the fixed scale 34 match. On the other hand, when the moving scale 3 moves by exactly xc2xd of the pitch of the optical grid from that state, the transparent portions and nontransparent portions of the optical grids overlap, so the amount of light transmitted becomes the minimum. That is, along with movement of the moving scale 3, the signals output from the photodiodes 28 become sinusoidal signals. By counting the number of their cycles, the distance of movement of the moving scale 3 can be found.
In general, the fixed scale 34 is normally provided with two optical grids 47 and 48. Corresponding to this, two photodiodes 28 are also provided. Further, one optical grid 47 is shifted by exactly xc2xc pitch from the other optical grid 48.
FIG. 3 shows the signals output from the two photodiodes 28 when the moving scale 3 moves. If expressing the light passing through one optical grid 47 of the fixed scale 34 as the signal A in FIG. 3, the signal B expressing the light passing through the other optical grid 48 of the fixed scale 34 is shifted in phase from the pitch P of the signal A by xc2xc pitch. It is possible to determine the right or left direction of movement of the moving scale 3 by the advance or delay of the phase of the signal B with respect to the signal A.
FIG. 4 shows the configuration of a first prior art of a photoelectric transmission type linear encoder built into the contact type digital displacement meter 40 explained in FIG. 1. A cross-section of the linear encoder is shown. The linear encoder is mainly provided with two LEDs 1 used as light sources, a moving scale 3, a spindle 5, a fixed scale 34, and two photodiodes 28.
The frame 8 has an upper cover 29A and lower cover 29B and a linear encoder support base 30 screwed to it. The spindle 5 is supported by two bearings 18 fastened to the frame 8. A contactor 6 for contacting the measured object is screwed into the front end of the spindle 5. The moving scale 3 is positioned with and fastened to a moving scale support base 31. The moving scale support base 31 is positioned with and fastened to the spindle 5, so movement of the spindle 5 becomes movement of the moving scale 3. The moving scale 3 is sandwiched between the light source LEDs 1 and condenser lenses 2 and the light receiving side fixed scale 34 and photodiodes 28.
For a stopping mechanism of the spindle 5, while not shown, a stopping rod is fastened to the spindle 5 at one end. The other end slides in a groove provided in the frame 8 to thereby function as a stop. Further, the rod is linked with the frame 8 by a tension spring and is set to apply a suitable contact pressure to the measured object.
At the light emitting side, the two LEDs 1 are fastened to the LED support base 32. The condenser lenses 2 are fastened to the LEDs 1. The LED support base 32 is positioned with and screwed to the linear encoder support base 30 so as to facilitate positioning with the light receiving side. The two LEDs 1 and condenser lenses 2 sandwich the moving scale 3 between them and face the light receiving side fixed scale 34 and two photodiodes 28.
At the light receiving side, the two photodiodes 28 are set on a PCB (printed circuit board) 33. The PCB 33 is fastened to the linear encoder support base 30. The fixed scale 34 is set on the linear encoder support base 30 between the photodiodes 28 and the moving scale 3. Two sets of gradations are cut into it. As with the explanation of the principle in FIG. 2, the pitches and line widths of the two optical grids 47 and 48 provided at the fixed scale 34 are exactly the same as the optical grid 11 of the moving scale 3, but the gradations are shifted in relative position by exactly xc2xc pitch corresponding to the two photodiodes 28.
When the spindle 5 moves and the moving scale 3 is moved due to measurement, the light from the LEDs 1 and condenser lenses 2 passes through optical grid 11 of the moving scale 3 to produce differences in lightness. When the transparent portions of the optical grid 11 match with the transparent portions of the optical grids 47 and 48 of the fixed scale 34, the light is bright, while when they are shifted in phase by 180xc2x0, the light becomes dark. The repetition of the differences in lightness of the light is detected by the photodiodes 28. As shown in FIG. 3, two sinusoidal signals A and B having the same period and having a 90 degree phase difference are output from the photodiodes 28 by the xc2xc pitch shifted optical grids 47 and 48 of the fixed scale 3. These signals A and B are amplified and digitalized, then electrically divided and output as 1 xcexcm pulses to enable measurement of the length.
FIG. 5 is a view of the configuration of a photoelectric transmission type linear encoder of a second prior art. The photoelectric transmission type linear encoder shown in FIG. 5 is comprised of a glass scale 10 (moving scale 3 of first prior art), an optical grid 11 provided on the glass scale 10, a light source 1 for emitting parallel light to the glass scale 10, fixed index grids 51 to 54 for receiving light passing through the glass scale 10, an index base 50 on which the fixed index grids 51 to 54 are provided, light receiving elements 61 to 64 for receiving the light passing through the fixed index grids 51 to 54, and a board 20 on which the light receiving elements 61 to 64 are provided. Further, the board 20 is provided with a semiconductor integrated circuit (IC) 22 and a terminal 21 for connecting with a cable 7C.
Note that the phases of the fixed index grids 51 to 54 are shifted in 90 degree increments with respect to the optical grid 11. Further, the light receiving elements 61 to 64 are comprised of single light receiving elements such as photosensors. The signals obtained are converted to length using the prior art of xe2x80x9cinterpolationxe2x80x9d for converting voltage to distance.
FIG. 6 is a view of the configuration of an optical transmission type linear encoder of a third prior art. The optical transmission type linear encoder shown in FIG. 6 is comprised of a glass scale 10, an optical grid 11 provided on the glass scale 10, a light source 1 for emitting parallel light to the glass scale 11, a light receiving element array 37 for receiving the light passing through the glass scale 10, and a board 20 on which the light receiving element array 37 is provided. Further, the board 20 is provided with a semiconductor integrated circuit (IC) 23 and a terminal 21 for connecting with a cable 70.
FIG. 7 will be used to explain the configuration of the optical transmission type linear encoder of the third prior art in further detail. The light receiving element array 37 is comprised of a plurality of light receiving elements. P shows the pitch of the light receiving elements, u shows the width of the valid light receiving portion 35, and r shows the width of the invalid light receiving portion. Here, P is set to Sxc3x973/4, u to S/2, and r to S/4. That is, the ratio of u and r is 2:1.
Therefore, four light receiving elements g1, g2, g3, and g4 are provided corresponding to the three optical grids e1, e2, and e3. Further, the light receiving elements are configured so as to give the same amount of light for every four elements. Further, the phases of the four light receiving elements g1, g2, g3, and g4 are shifted by 90xc2x0 increments. Therefore, lines are laid for each four light receiving elements and the values added. Here, the total of the added outputs from the valid light receiving portions a1, a2, a3 . . . of the light receiving elements 37 is designated as A, the total of the added outputs from the valid light receiving portions b1, b2, b3 . . . of the light receiving elements 37 as B, the total of the added outputs from the valid light receiving portions c1, c2, c3 . . . of the light receiving elements 37 as C, and the total of the added outputs from the valid light receiving portions d1, d2, d3 . . . of the light receiving elements 37 as D. This being so, the phases of the added output signals A, B, C, and D are shifted by 90 degree increments. The optical transmission type linear encoder of the third prior art measures length by processing the output signals A, B, C, and D. These output signals A to D are changed to length using the conventional interpolation technique.
In the above first prior art, however, since the difference in lightness due to the overlap of the moving scale 3 and the fixed scale 34 was detected by photodiodes 4, the fixed scale 34 was essential, the contact type digital displacement meter 40 could not be made thin, and therefore the contact type digital displacement meter 40 became large in size. Further, the distance between the moving scale 3 and the fixed scale 34 had to be made a narrow 10 to 50 xcexcm, therefore there was the problem that adjustment for positioning the surfaces of two scales with each other was extremely difficult.
Further, the photoelectric transmission type linear encoder of the second prior art was comprised by a combination of the glass scale 10, fixed index grids 51 to 54 corresponding to the fixed scale 34 in the first prior art, and light receiving elements 61 to 64. The fixed index grids 51 to 54 were essential, so the contact type digital displacement meter 40 became large in size. Further, to perform measurement with a high precision, it was necessary to accurately set the distance between the index grids, the pitch of the fixed index grids 51 to 54, the ratio of transparent portions and nontransparent portions of the fixed index grids 51 to 54, the distance between the glass scale 10 and the fixed index grids 51 to 54, and the distance between the fixed index grids 51 to 54 and the light receiving elements 61 to 64.
Further, the light receiving elements 61 to 64 were comprised of single light receiving elements such as photosensors, so it was difficult to arrange them close to each other, a wide area was occupied, and the efficiency of use of the portion which the light struck was lowered.
On the other hand, in the third prior art, the size of the light receiving elements had to be fixed to Sxc3x973/4 and the width r of the invalid light receiving portions was set to S/4. It was not possible to further lower this.
Further, as problems common to the second and third prior arts, there were the problems of how to widen the valid light receiving portions of the light receiving elements to raise efficiency at the portion struck by light and what measures to take when the illuminance of the light source was spotty or when the glass scale was scratched or dirty.
That is, if the areas of the valid light receiving portions of the light receiving elements are small at the portion struck by the light, the outputs become smaller, there is susceptibility to noise, and there is a detrimental effect on the measurement precision. Further, when the illuminance of the light source is uneven or spotty, the same light receiving sensor always gave values different from the normal values and there was a detrimental effect on the measurement precision. Further, when the glass scale 10 was scratched or dirty, the location of differences in illuminance would move along with movement of the glass scale, the light receiving elements receiving this would give erroneous values, and there would therefore be a detrimental effect on the measurement precision.
Therefore, a first object of the present invention is to provide a contact type digital displacement meter which eliminates the fixed scale for detecting the difference in lightness of an overlapping moving scale and fixed scale, makes the contact type digital displacement meter thinner, and facilitates adjustment for positioning the surfaces of two scales with each other.
A second object of the present invention is to enable the effects of any unevenness or spottiness of the illuminance of the light source or any scratches or dirt on the glass scale to be suppressed and prevent a detrimental effect on the measurement precision of an optical displacement measurement apparatus.
A third object of the present invention is to increase the areas of the valid light receiving portions of the light receiving elements at the portion struck by the light from the light source so as to raise the light receiving efficiency and to enable the effects of any unevenness or spottiness of the illuminance of the light source or any scratches or dirt on the glass scale to be suppressed and prevent a detrimental effect on the measurement precision of an optical displacement measurement apparatus.
To achieve the first object, in the case of an optical displacement measurement apparatus having a displaceable first member having an optical grid, a light source for emitting light to the first member, and a light receiving element unit for receiving light passing through the first member, the present invention is characterized by setting a distance between the first member and light receiving element units to xc2xd of a Talbot distance and comprising the light receiving element unit by light receiving element groups. Further, in the case of a photoelectric transmission type linear encoder comprised of a light source comprised of an LED and condenser lens, a first member comprised of a moving scale, and a photodiode masked at the same pitch as the first member, the present invention is characterized by setting a distance between the first member and the photodiode to xc2xd of a Talbot distance.
Here, the xe2x80x9cTalbotxe2x80x9d phenomenon is the phenomenon that the same distribution of light intensity is reproduced as on the surface of a cyclical structure at a distance (Talbot distance) of a whole multiple of the distance given by
Zt=(2xc3x97D2)/xcex
where D is the grid pitch and xcex is the wavelength, when emitting planar monochromatic light to a cyclical structure such as a diffraction grid and was discovered by H. F. Talbot in 1836.
According to this means, if the pitch of the moving scale is made 20 xcexcm and the wavelength xcex of the light emitting element is made 700 nm, the distance forming a Talbot image becomes
(2xc3x97D2)/xcex=Zt=1,142 xcexcm
Therefore, it is sufficient to set a light receiving element, that is, a photodiode, at a position of Zt/2=571 xcexcm. Compared with the past, the distance can be increased 10 to 500 fold, the ease of assembly can be improved, and an inexpensive contact-type digital displacement meter can be provided.
To achieve the second object, an optical displacement measurement apparatus of the present invention is an optical displacement measurement apparatus having a displaceable first member having an optical grid, a light source for emitting light to the first member, and a light receiving element unit for receiving light passing through the first member, characterized in that the light receiving element unit is comprised of light receiving element groups which are arranged shifted in increments of a predetermined distance with respect to a direction of displacement of the first member.
In this case, it is possible to configure the light receiving element groups by providing a plurality of light receiving element arrays provided at a predetermined pitch of the valid light receiving portions and the invalid light receiving portions. Further, it is possible to arrange the plurality of light receiving element arrays shifted by increments of a predetermined distance in the direction of displacement of the first member. The optical grid may be comprised of transparent portions and nontransparent portions provided at a pitch S of a ratio of the width of the transparent portions and the width of the nontransparent portions of 1:1. Further, the light receiving elements comprising the light receiving element arrays may be provided at a pitch S of a width of the valid light receiving portions and a width of the invalid light receiving portions of 1:1.
According to this configuration, since there are a plurality of light receiving elements having specific positional information and there are a plurality of dispersed light receiving element arrays comprised of a plurality of light receiving elements having this specific positional information, even when the illuminance of the light source is uneven or spotty or when the glass scale is scratched or dirty, all of the light receiving elements suffer that effect a bit each and average it out and therefore it is possible to prevent any detrimental effect on the measurement precision.
To achieve the third object, the optical displacement measurement apparatus of the present invention has a displaceable first member having an optical grid, a light source for emitting light to the first member, and light receiving element arrays for receiving light passing through the first member and having a plurality of light receiving elements at the valid light receiving portions and invalid light receiving portions, characterized in that the pitch of the optical grid is larger than the pitch of the light receiving elements and the light receiving element arrays are arranged in the direction of displacement of the first member.
In this case, it is possible to make the ratio of the width of the transparent portions and the width of the nontransparent portions of the optical grid of the first member 1:1 and make the width of the valid light receiving portions and width of the invalid light receiving portions of the light receiving elements such that the width of the valid light receiving portions is larger than the width of the invalid light receiving portions.
In addition, it is possible to make the width of the invalid light receiving portions 2 to 3 xcexcm.
Further, it is possible to provide light receiving element groups having pluralities of light receiving element arrays.
According to this configuration, since the width of the valid light receiving portions and width of the invalid light receiving portions of the light receiving elements are such that the width of the valid light receiving portions is larger than the width of the invalid light receiving portions, it is possible to increase the areas of the valid light receiving portions of the light receiving elements at the portion struck by light and possible to give resistance to noise and prevent a detrimental effect on the measurement precision.
Further, since a plurality of light receiving elements having specific positional information are arranged dispersed or a plurality of arrays comprised of a plurality of dispersed light receiving elements having specific positional information are arranged dispersed, even when the illuminance of the light source is uneven or spotty or when the glass scale is scratched or dirty, all of the light receiving elements of the positional information suffer that effect a bit and average it out and therefore it is possible to prevent any detrimental effect on the measurement precision.